The subject matter disclosed herein relates to the field of interventional cardiology, and more specifically to imaging techniques utilized in interventional cardiology procedures.
Interventional cardiology covers clinical procedures performed by physician for the sake of treating a coronary artery which is affected by a stenosis, i.e. a narrowing of the vessel. Due to this narrowing, blood supply to the heart/myocardium may be impacted. A common therapeutic option for this condition is to deploy a stent in the narrowed vessel. A stent is a metallic mesh structure which has a cylindrical form once deployed. This structure forces the vessel to return to a normal size due to the pressure exerted by the deployed stent and consequently the blood supply to the myocardium is hopefully restored.
This type of interventional cardiology procedure is done using a minimally invasive approach. In particular, the different interventional tools used in the procedure are guided or threaded to the place of interest in the artery/blood vessel under the guidance of images showing the structure and the surrounding tissue of the patient. The tools, normally in the form of a catheter, guidewire, and stent, are introduced at a puncture point, which could be the femoral artery or the radial artery. The images used by the physician to guide the catheter along the vessel to the desired/affected area are x-ray images which offer a field of view of several centimeters and depict all the anatomical elements which are inside this field of view. The contrast in these types of images depends on the x-ray absorption of the material by the x-rays. For example, the design/materials of the employed interventional tools makes them reasonably radio-opaque and easily visible in the x-ray images. For the anatomical tissue shown in the images, such as the vessel, this tissue can be more transparent making it potentially necessary to inject a contrast media to more readily visualize their structures in the patient.
In some cases, other imaging systems may be used as an alternative to x-ray images. The reasons for this can be that the physician may want to know some information on the vessel wall/structure to understand the root cause of the narrowing or to better define its extent within the vessel, which cannot be readily done with x-ray images. Indeed, it is common physiological knowledge that often the narrowing is due to a plaque which builds up on the vessel wall over time. The nature of this plaque may be variable, as it may be calcified, fibrous, etc. It is also known the first reaction of the vessel when a plaque starts to build up is to enlarge and thus attempt to maintain the lumen size. However, these details are not readily visible in x-ray image of the vessel, though they can be important in the treatment of the patient.
To address this issue, the industry has developed alternative and/or additional imaging modalities, such as Intra Vascular Ultra Sound (IVUS) or Optical Coherent Tomography (OCT). The general operation mode for these tools is to provide images from a signal obtained using a sensor navigated inside the artery itself. The images obtained from these modalities depict a slice of the tissue perpendicular to the longitudinal axis of the vessel. The location of this slice is precisely defined by the location of the sensor in the artery at the time when the signal was collected. In some other situations, a different sensor may be introduced in the artery such as a pressure sensor or a temperature sensor. In the cases using these types of sensors, the information collected is a single mono-dimensional signal, corresponding to the temporal variation of the signal collected by this sensor. In either configuration, the physician uses the images obtained using the sensors to position/reposition the catheter at the particular point of interest in the blood vessel for treatment.
The main steps of prior art interventional cardiac procedures employed using these known imaging techniques are the following:                Puncture the patient at the level of the femoral artery or radial artery,        Introduce a catheter and drive it under the guidance of x-ray images into the ostia of a coronary artery.        Perform some images of the coronary arteries with injection of contrast media.        Assuming that a stenosis area is revealed by these images and the physician decide that he wants to interrogate this vessel with intravascular imaging or intravascular sensor:                    Introduce a guide wire inside the catheter and push it to the ostia of the coronary. Once the guide wire gets out of the catheter, the wire is navigated along the vessel to reach the distal portion of the diseased coronary artery.            Slide along the guide wire, which is used as rail, the intravascular equipment including the sensor. The appearance of this equipment in the x-ray image is usually a small elongated (2 or 3 mm length, ⅔ of mm diameter) object.            Activate the imaging/recording capability of the intravascular equipment.            Push/pull the intravascular equipment to observe at different locations along the vessel. Most times but not exclusively, the intravascular equipment is pulled using motorized pullback device acting at constant speed.            In parallel with the use of the sensor, the operator may at his discretion, obtain some x-ray images which display the position of the intravascular sensor inside the vessel.                        Once the analysis of this data is finished, the physician can perform different actions such as deploying a stoat at the vessel section being identified as diseased, optimize the already deployed stent, etc.        
The primary medical purpose associated with the use of intravascular device/equipment is to relate the information collected on the vessel via the intravascular equipment/sensor to a precise location along the image of the vessel obtained using the x-ray. In attempts to effectively achieve this, several prior art strategies exist:                (1) Use the physician's experience in analyzing both the x-ray image and the intravascular data to relate/correlate the information. For example, the physician may use the location of the narrowing illustrated in the two images and from this extrapolate the correspondence between the image and a location along the vessel. This approach is feasible when the intravascular data has been collected using a motorized pullback mechanism. Assuming a fixed acquisition rate, it is possible to roughly relate position in the vessel and position in the stack of intravascular data.        (2) Use advanced technical means (i.e., a computer modelling program) to build up an image of the vessel with a centerline that corresponds to the center of the vessel using the sensor data. This centerline is established in the 3D space and so any distance measured along it translates into a given difference of time assuming a constant speed for the automatic pullback of the sensor in the vessel. To relate the intravascular data with a given location, the operator can then manually define a common reference point. For example, he can point out a bifurcation of the vessel which can be observed in the two datasets.        (3) Use the following method steps:                    Shoot x-ray (fluoroscopic low-dose) images while the probe is moved in the artery and the sensor images are obtained.            With an image processing algorithm, segment out the location of the probe in these images.            Report this position in a cine image done in the same configuration of, the x-ray imaging system. In reporting this position, the algorithm has to manage additional sources of errors such as cardiac motion and the breathing motion, as these two motions change the location of the vessel over time.                        
This third option presents a technical solution which is currently offered by some imaging manufacturers and is very attractive for use by physicians. However, it depends on reliable algorithms to perform the following analysis in the x-ray image:                Identification of a centerline for the vessels in the cine image,        Tracking of the location of the probe in the fluoroscopic images.In performing the analysis, these algorithms must accurately account for significant errors that can occur as a result of the motion of the vessel in the images. Thus, the complexity of the analysis and accompanying algorithms and device is quite high in order to achieve an accurate correspondence between the various images.        
In one other attempt to provide correlation between these types of images, it has been proposed in the prior art to add a localization sensor to the intravascular equipment. This localization sensor could then report the position of the probe along the vessel at any time. This information is provided in the same referential as the x-ray image of the vessel, and has the some of the same shortcomings with regard to the errors and algorithms required to overcome these errors for accurate images. With respect to the third option listed above, the challenging problem of tracking the location of the probe in the fluoroscopic images is avoided. On the other side from a technical complexity point of view, the inclusion of a localization sensor into the intravascular equipment is a significant difficulty. Moreover some other pieces of equipment shall be added to operate this sensor.
Hence it is desirable to provide a method and system for assessing a lesion or affected area of a blood vessel for treatment, including the nature of the lesion, the location of the lesion on the vascular tree, the dimension (e.g., length and diameter) of the lesion and tortuosity of the access to the lesion along the vascular tree that does not require complicated algorithms to correct for errors and/or fluctuations in the images obtained.